1. Field of the Invention
The present invention relates to an integrated fluorine gasket manufactured by injection molding for hydrogen fuel cells. More particularly, the present invention relates to an integrated high-fluidity/high-elasticity fluorine gasket manufactured by injection molding for hydrogen fuel cells that has excellent fluidity and excellent elasticity so that, when a fluorine gasket is integrated with a thin bipolar plate by injection molding on the thin bipolar plate, the thin bipolar plate may be minimally deformed and sealing durability of a stack is increased as a result.
2. Description of the Related Art
In general, polymer electrolyte membrane fuel cells (PEMFCs) are widely used as fuel cells for vehicles. Thus, once several hundreds of unit cells of PEMFCs are stacked together to be manufactured as a single contiguous stack, in order to mount the stack on a vehicle and to provide high output performance of about 100 kW in various operation conditions normally, PEMFCs should operate stably in a wide range of current densities.
During a reaction for generating electricity in the fuel cells, after hydrogen supplied to an anode that is an oxidation electrode in a membrane-electrode assembly (MEA) of the fuel cells is divided into protons (or hydrogen ions) and electrons, protons are moved to a cathode that is a reduction electrode through an electrolyte membrane, and electrons are moved to the cathode through an external circuit. Oxygen molecules, hydrogen ions, and electrons react in the cathode together to generate electricity and heat and simultaneously to generate water as a reactant product. (Hereinafter, a membrane-electrode assembly and an MEA are used and referenced together as a single unit.)
In order to maintain a seal with respect to hydrogen/air that are reactant gases and water in the stack of the fuel cells used for vehicles, a gasket should be used in each cell. Thus, the gasket can be independently and separately manufactured so that it can be mounted in the fuel cell. Alternatively, the MEA and the gasket can be integrated with each other, or the gasket can be integrated with a bipolar plate by injection molding the gasket on the bipolar plate.
In order to produce a stack of fuel cells on a large scale efficiently, components should be rapidly assembled and stacked. In a large stack system, such as a hydrogen fuel cell vehicle, it is preferable to use a bipolar plate-integrated gasket having excellent handling properties and excellent heat-resistant properties during cross-linking of the gasket.
FIG. 1 is a perspective view of a conventional process of assembling a stack in which existing bipolar plate and gasket are not integrated with each other. As illustrated in FIG. 1, a conventional catalyst-coated membrane (CCM) type MEA that is configured by coating both sides of a membrane with catalysts of two electrodes, such as an anode and a cathode, and by bonding the catalysts of two electrodes is provided.
However, in a conventional stack assembling process, a CCM MEA 1, a gas diffusion layer 2, a gasket 3, and a bipolar plate 4 should be bonded to one another so that the number of working processes and a working time can be increased. In addition, since these stack components are not in the form of an integrated module, most of the assembling process is performed manually. Thus, when the arrangement of the components is not precise or non-uniform, the entire cell performance and durability may be lowered after a fuel cell stack is assembled.
In order to solve the problems of the conventional stack assembling process, a process of assembling a fuel cell stack by using a structure (3+4) that is formed by integrating a gasket 3 and a bipolar plate 4 prior to assembling, as illustrated in FIG. 2 has begun to be widely used. When the gasket 3 is used after being integrated with the bipolar plate 4 by injection molding on the bipolar plate 4, an individual gasket does not need to be additionally attached to the bipolar plate 4 during stack assembling so that the stack assembling process is more convenient and mass productivity can be increased.
A gasket used for a fuel cell stack for hydrogen fuel cells should satisfy various requirement properties, such as proper hardness, superior elasticity or a very low compression set, excellent mechanical properties, excellent resistance to acid/resistance to hydrolysis, low diffusion with respect to hydrogen/air (or oxygen)/coolant, a low content of impurities that may cause catalyst poisoning, excellent thermal resistance, high electrical insulation, high productivity, and low cost. Polymeric elastomers that are mainly used in a gasket for a stack for hydrogen fuel cell vehicles that satisfy these requirement properties, may be largely classified into fluorine-based elastomers (or fluoroelastomers), silicone elastomers, and hydrocarbon elastomers.
The fluorine-based elastomers are largely classified into FKM and FFKM by American Society for Testing and Materials (ASTM) and are widely used in various fields, such as car/construction/petrochemicals industries. The silicone elastomers are largely classified into general-purpose silicone elastomers, such as polydimethylsiloxane, and modified silicone elastomers, such as fluorosilicone. Solid silicone elastomers can be used. However, liquid silicone rubber has been more widely used for precise injection molding for fuel cells. In addition, the hydrocarbon elastomers, such as ethylene-propylene diene monomer (EPDM) and ethylene-propylene rubber (EPR), have been used.
The fluorine-based elastomers in particular, which have excellent elasticity, excellent resistance to acid and heat, are often used for applications that are subjected to long term severe operation conditions, such as hydrogen fuel cell vehicles and have received much attention as a gasket for stack applications.
In particular, the fluorine-based elastomers show low polarizability and unique properties caused by strong electronegativity of fluorine atoms. When the fluorine content of the fluoroeleastomers is large, the fluorine-based elastomers show characteristics, such as high thermal/chemical/ageing resistance, weather resistance, excellent chemical resistance with respect to solvent/hydrocarbon/acid/alkali, low dielectric constants, low flammability, low surface energy and low moisture absorption. In addition, C—F bonds contribute to high resistances to both oxidation and hydrolysis.
However, conventional commercialized fluorine-based elastomers have high molecular weights and high melt viscosity and thus are not appropriate to form into complicated shapes quickly and easily. Thus, for the most part, fluorine-based elastomers are used in compression molding, transfer molding, extrusion, and calendering rather than precise injection molding.
In particular, in order to manufacture a gasket for hydrogen fuel cell vehicles by injection molding the gasket on a thin bipolar plate by using over-molding, as described above, an injection molding processability of a fluorine compound based on fluoroelastomers must be very excellent. As it currently stands, this is not the case.
To this end, fluidity of the fluorine compound should be high, and pre-curing or scorch should not occur during cross-linking, and in order to guarantee 10-year durability for hydrogen fuel cell vehicles, a gasket for a stack is required to exhibit high elasticity to secure sealing durability.